Compound oxide high temperature superconducting material and a method for preparing same

ABSTRACT

A superconducting material and a process for producing a superconducting material comprising a compound oxide represented by the general formula: 
     
         (Ba, γ).sub.x (α,β).sub.1 -.sub.x ε.sub.y 
    
      Cu 1-y  O 3  -z 
     in which 
     &#34;γ&#34; represents an element of the IIa group of the periodic table except Ba, an atomic ratio of γ to Ba, being selected in a range between 1% and 90%, 
     &#34;αrepresents Y or La, 
     &#34;β&#34; represents an element of the IIIa group of the periodic table but is different from α, an atomic ratio of β to α being selected in a range between 1 and 90%, 
     &#34;ε&#34; represents a metal element of the IIIb group of the periodic table, 
     x, y and z are numbers each satisfies ranges of O≦x≦1, O≦y≦1, and O≦z≦1 respectively, and 
     the expression of (Ba, γ) and (α, β) mean that the respective elements occupy predetermined sites in a crystal in a predetermined proportion.

This is a continuation of application Ser. No. 07/426,754, filed Oct.26, 1989, now abandoned, which is a division of application Ser. No.07/223,634, filed Jul. 25, 1988, now U.S. Pat. No. 4,880,773.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a superconducting material and a methodfor preparing the same. More particularly, it relates to a novelsuperconducting material composed of compound oxide having a highcritical temperature and a method for preparing the same.

2. Description of the Related Art

The phenomenon of superconductivity is understood in the art as acondition wherein perfect diamagnetism is observed and no difference inpotential is observed and wherein an electric current of a constantfinite value is observed internally in the superconducting medium.

Superconductivity and superconducting materials can be utilized in thefield of electrical power applications such as MHD power generation,fusion power generation, power transmissions, electrical powerreservation or the like; in the field of transportation, for example,magnetic levitation trains, magnetically propelled ships or the like; inthe medical field, for example, in a high-energy beam radiation unit; inthe field of science, such as in NMR or high-energy physics; for highlysensitive sensors or detectors for sensing very weak magnetic fields,microwave, radiant rays or the like, or in the field of fusion powergeneration. In addition to the above mentioned electrical powerapplications, superconducting materials can be used in the field ofelectronics, for example, as a device using the Josphson device which isexpected to be a high-speed and low-power-consuming switching device.

The phenomenon of superconductivity, however, has heretofore beenobserved only at very low cryogenic temperatures. In fact, a relativelylow temperature of 23.2° K., which was the critical temperature (Tc) ofa superconductor composed of Nb₃ Ge has until recently been the highestrecord critical temperature among known superconducting materials.

At such temperature, liquidized helium (boiling point of 4.2° K.) is theonly cryogen which can be employed to realize this very low criticaltemperature in the superconducting material. However, helium is not onlya limited, costly resource, but also requires a large-scale system forliquefaction. Therefore, there had been a strong demand for othersuperconducting materials having higher critical temperatures (Tc),however no materials having critical temperatures exceeding the abovementioned Tc had been discovered in approximately the past ten years.

Reports of the existence of a new type of superconducting materialhaving a much higher Tc were published by Bednorz and Muller, whodiscovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986)189]. It has also been reported in February 1987 that C. W. Chu et aldiscovered, in the United States of America, another high-temperaturesuperconducting material, termed a YBCO type, of the form YBa₂ Cu₃ O₇-x, having a critical temperature of about 90° K. Hence, much activityin the field of high-temperature superconductors has taken place in theensuing months.

It had been known that certain compound oxide ceramic materialsexhibited the property of superconductivity. For example, U.S. Pat. No.3,932,315 discloses a Ba-Pb-Bi type compound oxide which exhibitssuperconductive properties and Japanese patent laid-open No. 60-173,885discloses that Ba-Bi type compound oxides also exhibit superconductiveproperties. These superconducting materials, however, possess rather lowtransition temperatures of about 10° K. and hence usage of liquidizedhelium (boiling point of 4.2° K.) as a cryogen had been indispensable inorder to cool the material sufficiently to realize superconductivity.Therefore, the above mentioned new type compound oxides in whichsuperconductivity is realized at temperatures attainable using liquidnitrogen, which is a relatively cheap cryogen, will accelerate actualusage of superconductors.

An object of the present invention is to provide a new system ofcompound oxides which possess a higher critical temperature and a methodfor preparing the same.

SUMMARY OF THE INVENTION

A superconducting material according to the present invention iscomposed of a compound oxide represented by the general formula:

    (Ba, γ).sub.x (α, β).sub.1-x ε.sub.y Cu.sub.1-6 O.sub.3-z

in which

"γ" represents an element of the IIa group of the periodic table exceptBa, an atomic ratio of with respect to Ba is selected in a range between1% and 90%,

"α" represents Y or La,

"β" represents an element of the IIIa group of the periodic table but isdifferent from α, an atomic ratio of β with respect to α is selected ina range between 1% and 90%,

"ε" represents a metal element of the IIIB group of the periodic table,

x, y and z are numbers which satisfy ranges of O≦x≦1, O≦y≦a, and O≦z≦1respectively, and

the expression of (Ba, ≦) and (α, β) means that each of these elementsoccupies a predetermined site in a crystal in a predeterminedproportion.

A process for producing a superconducting material according to thepresent invention comprises preparing a material powder, compacting thematerial powder and then subjecting the resulting compact to a finalsintering operation, the process being characterized in that thematerial powder is selected from a group comprising one of the followinggroups (A), (B) and (C):

(A) a powder mixture composed of powders selected from a groupcomprising:

(i) powders of elements Ba, Cu, α, β, γ and εe and

(ii) powders of compounds each containing at least one of said elementsBa, Cu, α, β, γand ε;

(B) a sintered powder obtained by sintering preliminarily the powdermixture (A) and then pulverizing the sintered mass; and

(C) a powder mixture of said powder mixture (A) and said sintered powder(B),

in which, "γ" represents an element of the IIa group of the periodictable except Ba, "α" represents Y or La, "β" represents an element ofthe IIIa group of the periodic but is different from α, and "ε"represents a metal element of IIIb group of the periodic table,

wherein, an atomic ratio of (Ba, γ): (α, β): ε: Cu in said materialpowder satisfies a ratio of x : (1-x) : y : (1-y), in which an atomicratio of γ to Ba is selected in a range between 1% and 90%, an atomicratio of β to α is selected in a range between 1% and 90%, x and y arenumbers each satisfying ranges of O≦x≦1 and O≦y≦1 respectively.

The sintered compact obtained by the above mentioned process is composedof a so-called quasiperovskite type compound oxide which is representedby the general formula:

    (ba, γ).sub.x (α, β).sub.1-x ε.sub.y Cu.sub.1-6 O.sub.3-z

in which

"γ" represents an element of the IIa group of the periodic table exceptBa, an atomic ratio of γ with respect to Ba being selected in a rangebetween 1% and 90%,

"β" represents an element of the IIIa group of the periodic table but isdifferent from α, an atomic ratio of β with respect to α being selectedin a range between 1% and 90%, represents a metal element of the IIIbgroup of the periodic table,

x, y and z are numbers each satisfying ranges of O≦x≦1, O≦y≦1, and O≦z≦arespectively, and

the expression of (Ba, γ) and (α, β) means that each of these elementsoccupies a predetermined site in a crystal in a predeterminedproportion.

The atomic ratio of γ with respect to Ba can be selected in a rangebetween 1% and 90% but a preferred result is obtained in a range between1% and 50%. Further, there is a tendency that preferable properties assuperconductors are obtained when the value of x is selected in a rangeof 0.3≦x≦0.4.

The essence of the present invention resides in that the compound oxideis composed of the above mentioned specified elements and oxygen. Aunique point of the present invention, in comparison to theabove-mentioned known new type superconductors composed of four elementsis that the compound oxide according to the present invention has twopairs of elements, each pair of which belongs to the same group of theperiodic table. It is believed that an advantageous result of thepresent invention is obtained when each of these elements occupies apredetermined site in the crystal in a predetermined proportion .

In order to obtain a sintered compound oxide possessing superiorsuperconducting properties, it is indispensable to control the followingfactors:

(i) particle size of the material powder,

(ii) temperature during the preliminary sintering operation,

(iii) particle size of the preliminary sintered and pulverized powder,and

(iv) temperature during the final sintering operation.

In fact, if an average particle size of the material powder exceeds 10μm, a fine particle which is satisfactory in uniformity can not beobtained easily even after such material powder is subjected to thepreliminary sintering operation. Therefore, the average particle size ofthe material powder is preferably less than 10 μm.

In the case where a sintered powder (B) which is prepared by preliminarysintering of the powder mixture (A) is used as the material powder, thepreliminary sintering is preferably carried out at a temperature whichis higher than 700° C. but is not higher than the lowest melting pointof any compound which is contained in the powder mixture (A), for aduration of from 1 to 50 hours and preferably in an atmospherecontaining oxygen gas of 10-3 to 10² Torr. It is also preferable that,after the preliminary sintering is complete, the resulting sintered massis cooled at a cooling rate which is not slower than 1° C./min but nothigher than 1,000° C./min. A particle size of the pulverized powderobtained from the preliminary sintered mass directly influences theparticle size of a crystal which is obtained after the final sinteringoperation, so the preliminary sintered mass is preferably pulverized topowder having an average particle size of less than 5 μm. Such fineparticles increase the area of grain boundaries, which is one of thecritical factors for realizing the superior superconductor which isparticularly improved in the critical temperature of superconductivity.Pulverization or reduction of the preliminary sintered mass to particlesless than 1 μm in size is not only not economical or practicable butalso increases the possibility of contamination of the material powder.Therefore, the average particle size of the preliminary sintered powderis preferably adjusted to a range between 1 μm and 5 μm.

A series of operations of preliminary sintering, pulverization andcompacting is preferably repeated several times to homogenize thematerial powder.

The sintering temperature is one of the critical factors in the processfor producing the superconducting compound oxide, therefore thesintering temperature is controlled and adjusted to a temperature atwhich the sintering proceeds in a solid reaction without substantialfusing of the material powder and excessive crystal growth does notoccur in the resulting sintered compound oxide. In fact, the sinteringtemperature should not exceed the lowest melting point of any componentin the material powder such as the powder mixture, the compound powderor the preliminary sintered powder. Conversely, satisfactory sinteringcan not be effected if the sintering temperature is too low. Therefore,the sintering must be carried out a temperature which is higher than700° C. The duration of the sintering operation is generally for 1 hourto 50 hours in actual practice, although a longer sintering time ispreferable.

The above-mentioned preliminary sintering operation should also beprecisely controlled in the same manner as above for the same reason.

According to a preferred embodiment, the sintered compound oxide isfurther heat-treated in order to homogenize the crystal structure. Thisheat-treatment improves the critical temperature and reduces remarkablythe discrepancy between the terminal temperature of phase change whereperfect zero resistance is observed and the critical temperature.

This heat-treatment is preferably carried out in an oxygen containingatmosphere at a temperature of 500° to 900° C. Under this condition ofheat-treatment, the crystal structure of the sintered compound oxide isstabilized and the oxygen deficient perovskite structure which isdesired for realizing the superconductivity is obtained, the resultbeing that the lower critical temperature where perfect zero resistanceis observed becomes much higher and a lasting and stable superconductoris assured. If the temperature of heat-treatment is not higher than 500°C., the above-mentioned effect is not obtained or it takes a longer timebefore the objective crystal structure is realized. Conversely, if thetemperature of heat-treatment exceeds 900° C., the above-mentionedperovskite type crystal structure is lost.

The superconducting material according to the present invention can beused in the form of a sintered mass or article as is and may also beused in a form of a powder which is prepared by pulverizing the sinteredmass. This powder-formed superconducting compound oxide can be used forproducing a superconducting wire or the like. For example, thesuperconducting compound oxide powder according to the present inventionmay be compacted in a metallic pipe which is then drawn into a fine wireor may be mixed with suitable binder such as polyvinylbutylal (PVB) toprepare a paste which can be molded into a desired configuration orwhich is coated or applied in a desired pattern. The resulting wire orthe molded or coated paste are then sintered a final time.

When the superconducting compound oxide according to the presentinvention is used in the form of a paste, it is preferable to heat thecoated or molded paste at 400° to 700° C. in air before the finalsintering operation in order to remove the binder.

When a self-supporting article is molded with the paste, it ispreferable to select a thickness of the paste to be molded to be lessthan 1.2 mm, and when a thick film is coated on a support, the thicknessof the layer of paste is adjusted to be less than 0.6 mm, because apre-form molded with the paste by a doctor blade coating technique orextrusion technique will shrink during the final sintering stage so thatthe dimensions of the final product become smaller. Therefore, when thepaste is shaped into a form of a thick film or a self-supporting tape orwire, the thickness or diameter of paste coated or molded is preferablycontrolled to less than 0.6 mm and 1.2 mm.

The superconducting material according to the present invention can beformed into a thin film by a conventional physical vapor deposition(PVD) technique such as vacuum deposition, sputtering, ion-plating ormolecular beam epitaxial growth technique in the presence of oxygen gas.In this case, the above-mentioned sintered mass or block is used as avapor source or target which is evaporated in a vacuum chamber toproduce a thin film deposited on a substrate. Namely, the vapor sourcemay be a block prepared by sintering the material powder such as (i)powders of metal elements of Ba, Cu, α, β and ε (elements α, β and εhave the same definition as above) as they are, (ii) a powder mixture ofcompounds of these elements or (iii) their combination. The vapor sourcemay be a powder which is obtained by pulverizing the sintered block. Theproportion or atomic ratio of the elements in the vapor source isselected in such a manner that the desired atomic ratio of the elementsis realized in the deposited thin film taking into consideration thevaporization rate or sputtering rate or the like. The oxygen pressure inthe vacuum chamber should be controlled so that the partial pressure ofoxygen gas is adjusted to a range between 10⁻⁶ and 10⁻² Torr. Thesubstrates on which the thin film is deposited are preferably those thathave similar crystal structure or lattice constant and may be a singlecrystal of MgO, sapphire or SrTiO₃. Desirably, the superconducting thinfilm is deposited on a {001} plane or {110} plane of a single crystal ofMgO or SrTiO₃ to improve the critical current density (Jc) owing to theordering of the crystal to the c-axis.

In the case of thin film also, the above-mentioned heat treatment of thedeposited thin film is very effective. The heat treatment is carried outat a temperature of 500° to 900° C. for more than 1 hour in an oxygencontaining atmosphere in which the partial pressure of oxygen isadjusted to 10⁻³ to 10² Torr. After the heat treatment, the resultingthin film is cooled slowly at a cooling rate of less than 10° C./min.

The superconducting materials according to the present invention show avery high critical temperature compared to the conventional materials. Apossible reason for this is the characteristic features of thecomposition and the uniform and fine crystal structure which is assuredby the present process.

The novel superconducting materials according to the present inventionhave improved stability and a high critical temperature, so thatsuperconductivity may be attained using a relatively less expensivecryogen in a small liquefication system, and hence the superconductivematerials can be utilized advantageously in applications, such assuperconducting wire, rod, parts such as magnets, thick film or thinfilm devices, such as Matisoo switching elements or Josephson devices,Anacker memory devices, a variety of sensors, or Superconducting QuantumInterference Devices (SQUID).

Now, embodiments of the process according to the present invention willbe described by Examples, but the scope of the present invention is notto be limited thereto.

EXAMPLES

Powders of commercially available CuO and carbonate powders of Ba, Ca,La, Y and Tl, namely BaCO₃, CaCO₃, La₂ (CO₃)₃, Y₂ (CO₃)₃ Dy₂ (CO₃)₃, Tl₂CO₃ are mixed in an atomic ratio that satisfies the following generalformula:

    (ba, Ca).sub.x (α, Dy).sub.1-x Tl.sub.y Cu.sub.1-y

in which,

α represents La or Y, the value of x and y are shown in Table 1, and theproportion (%) of Ca with respect to Ba and the proportion (%) of Dywith respect to α are also shown in Table 1.

The respective powder mixtures are pulverized in a ball mill to bereduced to an average particle size of 4 μm and then sintered at 750° C.for 15 hours. The resulting sintered mass or cake is pulverized again.The steps of sintering and pulverization are repeated two times underthe same condition as above. The finally obtained powder having anaverage particle size less than 4 m is charged in a rubber mold andcompressed hydrostatically under a pressure of 1.5 ton/cm² to obtaintablets of 4×10×30 mm. The tablets are sintered at 950° C. for 8 hoursin air.

Resistance is measured on the tablets on which electrodes are securedwith silver electroconductive paste to determine the criticaltemperature.

Measurement of an upper critical temperature Tc and a lower criticaltemperature Tcf is effected by a conventional four probe method.Temperature is measured by a calibrated Au(Fe)-Ag thermocouple.

The results are shown in Table I.

                  TABLE I                                                         ______________________________________                                        Sample                T1    Ca   Dy                                           No.    α                                                                              x       (y)   %    %     Tc    Tcf                              ______________________________________                                        1      Y      0.66    0.11  10   30    125   112                              2      La     0.66    0.11  10   30    124   111                              ______________________________________                                    

We claim:
 1. A superconducting material comprising a compounded oxiderepresented by the general formula:

    (Ba, {γ}Ca)hd x(α, β).sub.1-x {ε}Tl, Cu.sub.1-y O.sub.3-z

excluding the following system:

    (ba, Ca).sub.x (α, Dy).sub.1-x Tl.sub.y Cu.sub.1-6 O.sub.3-z ;

wherein the atomic ratio of Ca to Ba is between 1% and 90%; "α"represents Y or La; "β" represents an element selected from the groupconsisting of Sc, Ac and lanthanidse, the atomic ratio of β to α beingbetween 1% and 90%; x, y and z are within the ranges of 0<×<1, 0<y<1,and 0≦z≦1 respectively; and the expression of (Ba, Ca) and (α, β) meansthat the respective elements occupy predetermined sites in a crystal ina predetermined proportion.
 2. A superconducting material according toclaim 1 wherein the atomic ratio of Ca to Ba is between 1% and 50%.
 3. Asuperconducting material according to claim 1 wherien saidsuperconducting material has perovskite type or quasi-perovskite typecrystal structure.
 4. A superconducting material according to claim 1wherein said superconducting material is in bulk form.
 5. Asuperconducting material according to claim 1 wherein saidsuperconducting material is prepared from paste comprising a powder ofsaid compound oxide mixed with polyvinylbutylal as a binder.
 6. Asuperconducting material according to claim 1 wherein saidsuperconducting material is in the form of a thin film deposited on asubstrate having a crystal structure or lattice constant approximatelythe same as that of said superconducting material.
 7. A superconductingmaterial as defined in claim 1, wherein the value of x is in the range0.3≦×≦0.4.